1. Field of the Invention
Embodiments of the present invention generally relate to spatial light modulator devices, and more particularly to a display system and method of using one or more fast response light sources, such as solid state light sources, and one or more spatial light modulator devices to improve the contrast ratio of the display system.
2. Description of the Related Art
Spatial light modulator (SLM) devices have numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, three-dimensional visual displays, holographic storage, microscopes, spectroscopes, medical imaging, and electrophotographic printing. A micro-mirror device (MMD) is one example of a SLM device. An MMD display typically comprises an array of mirrors in which each mirror can be electronically controlled to assume two positions—an “ON” state and an “OFF” state. Mirrors in an ON state reflect incident light to a projection lens onto a screen to form an image. Mirrors in an OFF state reflect incident light to a beam dump and do not reflect incident light to the projection lens. The brightness or intensity in an MMD display may be manipulated by controlling the time that a mirror spends in the ON state and in the OFF state during an image frame. Pulse width modulation (PWM) is one technique to control the time each mirror spends in the ON state during each frame time.
FIG. 1 is a bit-block representation of one example of a binary weighted PWM scheme in which the light intensity of a frame is controlled by splitting the frame into eight binary weighted time periods (B7-B0). The length of each block represents the amount of time the bit is asserted on an SLM, such as the amount of time a mirror of an MMD display is in the ON state. The length of time period corresponding to block B0, also called the least significant bit (LSB), is set at a predetermined value. The duration of the time period corresponding to B1 or the next significant bit is twice as long as that corresponding to the LSB. The duration of the time period corresponding to B2 is twice as long as that corresponding to the B1 and so on and so forth. Thus, the length of the time period corresponding to B7 (also called the most significant bit (MSB)) is 128 times the time period of the LSB. This gives a total of 256 possible intensity steps from zero intensity or full dark (where a mirror in an MMD display remains in the OFF state for the full frame time) to full intensity or full light (where a mirror in an MMD display remains in the ON state for the full frame time). U.S. Pat. No. 6,326,980 and U.S. Pat. No. 6,151,011 disclose other PWM schemes, the entirety of each being incorporated herein by reference.
One problem with MMD displays is producing good black levels due to the leakage of light. Therefore, a true “black” is difficult to provide; rather, black tones tend to appear gray. One issue that exacerbates this problem is that many MMD displays attempt to increase the total maximum brightness by providing a brighter light source. As a consequence, more light energy tends to leak from the display at zero normalized intensity, inhibiting even more the ability to produce a “true” black. Thus, these MMD displays provide poor contrast ratio and provide poor black levels.
One attempt to solve this problem has been the use of an iris aperture in the light path to attenuate the light to improve the contrast ratio. However, due to the slow speed of the mechanical iris aperture, the light can only be attenuated over an entire frame, including each of the primary color fields in the frame. One problem with this technique is that if any of the primary colors requires a high intensity of light, the light cannot be attenuated for the other color fields.
As the foregoing illustrates, there is a need in the art for an improved spatial light modulator devices and method of operating the same to improve the contrast ratio of the display system.